The pair, who after six years together still continue dating amid growing speculation of their being a royal engagement, were spotted in the Caribbean frolicking aboard a speedboat with pals.

Middleton, 26, waterskiied and relaxed in a black and white bikini, while Wills, also 26, chilled in blue board shorts.

"They only planned on a week there, which is a bit unusual, as they still have some time together before he continues military training," a royal insider tells PEOPLE.

The two hooked up amid the luxurious surroundings of a villa on Mustique, an exclusive West Indies island. According to Britain's Mail on Sunday, the couple spent a week at a villa owned by a Venezuelan millionairess.

Security protection was heavy for the vacationers. In addition to his usual bodyguards, William had the added protection of a local coast-guard vessel that cruised the waters – and rumbled at at least one boatload of paparazzi, PEOPLE has confirmed.

Now believed to be back in Britain, the pair are expected to socialize further.

"Because he has been away for quite a while now there will be lots of catching up with their group of friends in England," says the insider. "Summer is still the time of fun."